Phase-matched frequency conversion below 150 nm in KBe₂BO₃F₂.

Sum frequency mixing has been demonstrated below 150 nm in KBeBO₃F₂ by using the fundamental with its fourth harmonic of a 6 kHz Ti: sapphire laser system. The wavelength of 149.8 nm is the shortest ever obtained to our knowledge by phase matching in nonlinear crystals. The output powers were 3.6 μW at 149.8 nm and 110 μW at 154.0 nm, respectively. The phase matching angles measured from 149.8 to 158.1 nm are larger by 3-4 degrees than those expected from the existing Sellmeier equation. The measured transmission spectra of KBeBO₃F₂ crystals support the generation of coherent radiation below 150 nm.